1. Field of the Invention
The addition of certain sulfated and sulfonated materials to the aqueous streams of a multiplicity of units in the chemical processing industry will inhibit the fouling of process hardware. These materials work by dispersing the foulant material in the stream and preventing deposition of the foulant throughout the system. The usefulness of this invention is illustrated by its application in ethylene plant caustic systems, styrene monomer production, acrylonitrile recovery systems, and the terephthalic acid purification process.
2. Description of the Prior Art
In ethylene plants, hydrocarbon feedstocks are thermally cracked with steam to produce ethylene. Other hydrocarbon species are also produced along with less desirable impurities. Among these impurities are carbon dioxide and hydrogen sulfide; the so-called acid gases. These are removed from the cracked gas stream in a caustic scrubbing tower. Water soluble salts, Na.sub.2 CO.sub.3 and Na.sub.2 S, are formed and are removed in a water separator.
Another by-product produced in the cracking furnace is acetaldehyde. It arises from partial oxidation of ethylene. Due to its physical properties, acetaldehyde is carried with the cracked gas stream to the caustic scrubber tower. In this tower, acetaldehyde reacts with sodium hydroxide (caustic) to produce a homopolymer. This polymer is formed by self-condensation of acetaldehyde via the Aldol reaction. As the polymer grows, it becomes progressively less and less soluble in caustic. It eventually precipitates from solution and coats trays and other tower internal surfaces. Eventually, scrubbing efficiency is lost and the tower must be shut down and cleaned.
One method of dealing with this problem is described in U.S. Pat. No. 5, 160,425. This patent discloses use of carbohydrazide to derivatize acetaldehyde. This derivative will no longer react with caustic, and hence polymerization is stopped. Other compounds have been disclosed for this purpose as well. These are ethylenediamine, hydroxylamine salts and ethyl acetoacetate. Each of these compounds must be used in a stoichiometric ratio to the amount of acetaldehyde present. This is a costly method, however, and other more cost effective ways of treating this problem are continually being sought.
Lignosulfonates have been used for over 25 years to disperse polymer of this nature into caustic systems. The exact structure of lignin is not known, but the basic subunit of the polymeric structure is phenylpropane. The water soluble derivative, lignosulfonate helps prevent fouling by inhibiting deposition of the homopolymer onto process hardware. We have discovered that other, water-soluble dispersants will disperse polymeric acetaldehyde in caustic. Since it was not previously known that these materials would perform as dispersants for this system, the present invention represents a novel technology for this application.
A process used to purify terephthalic acid is described in volume 17 of the Encyclopedia of Chemical Technology. In this process, crude terephthalic acid is mixed with water to form a slurry. This slurry is passed through a heat exchanger and into a vessel called a dissolver. In the dissolver, the slurry is heated to a temperature greater than 250.degree. C. under enough pressure to keep water in the liquid phase. Under these conditions, terephthalic acid and its impurities are soluble in water.
In practice, this process leads to fouling of the preheat exchanger. Deposit analyses of samples from the exchanger indicate that the foulant is terephthalic acid. This means that a small amount of product is not being held in the slurry and is simply being deposited on the heat exchanger.
This problem is not successfully treated at this time. Even though terephthalic acid is an organic compound and not normally soluble in aqueous systems, it exists as an aqueous slurry until it reaches the dissolver. Therefore, any treatment must consist of a method of keeping terephthalic acid suspended in water. Thus, the addition of a water-soluble additive, capable of dispersing organic material into water, would be useful for the terephthalic acid process.
In a styrene manufacturing process, ethylbenzene and steam are fed into a reactor. Ethylbenzene is dehydrogenated to form styrene in a catalytic process. The temperature is very high, reaching temperatures in excess of 550.degree. C. From the reactor, crude styrene (containing unreacted ethylbenzene, steam and polymer) is cooled by a series of heat exchangers and enters an accumulator where condensed water and hydrocarbon are separated. Hydrocarbons flow out the top of this separator and are sent to the recovery section. Water flows out the bottom of this vessel and is sent to a hydrocarbon stripping tower where residual crude styrene is sent back to the separator. Water exits the bottom of this tower and is convened to steam for use in the reactor.
In the heat exchanger system, the condensation patterns are such that polymer precipitates from the gas stream first. It comes in contact with the exchanger walls and adheres to them. Water is next to condense followed by crude styrene. Thus, crude styrene is unable to redissolve precipitated polymer in this system because of the aqueous interface, leading to fouling on the heat exchangers.
This application is currently treated with an antioxidant. It is injected at the high temperature end of the heat exchanger train to help control formation of the polymer. However, some polymer is still formed. Addition of a dispersant would help move this polymer from the exchanger surface to the hydrocarbon layer and greatly improve operation of this unit.
In one method for the manufacture of acrylonitrile, gaseous reactants from the gas phase ammoxidation of propylene are cooled from an initial temperature of about 400.degree.-510.degree. C. and are passed countercurrent to an aqueous stream of acid such as sulfuric acid, to neutralize and recover any ammonia present such as disclosed in U.S. Pat. No. 3,404,947 and U.S. Pat. No. 3,408, 157. The resultant gases which contain major amounts of nitrogen and acrylonitrile and minor amounts of hydrogen cyanide, acetonitrile, carbon dioxide, carbon monoxide, propylene, ammonia, water, oxygen, acrolein and certain other carboxylic acids, aldehydes, and nitriles, are contacted with water at a temperature of 1.degree.-40.degree. C. to form a solution containing less than about 10 percent by weight acrylonitrile. The acrylonitrile (along with some water and hydrogen cyanide) is separated from any acetonitrile present by distillation and recovered overhead. Volatiles are separated from the resultant aqueous stream in a stripper. The bottoms from the stripper contain approximately 1 percent organic material as well as water-soluble polymers such as polyacrylic acid and its salts. These materials foul the surfaces of heat exchangers in the system, resulting in decreased production efficiencies. No technologies are currently practiced to alleviate this problem. The addition of an additive to disperse foulant material in the process would greatly improve the production of acrylonitrile by extending the time between cleanings of heat exchangers.